Spin-on-glasses (SOGs) are known materials used in the fabrication of semiconductor devices. SOGs are primarily used to planarize a semiconductor wafer, but are also used as interlayer dielectrics, passivation layers, and sacrificial etch-back layers. As the name "spin-on-glasses" suggests, SOGs are deposited onto a semiconductor substrate and the substrate is spun at high speeds to distribute the SOG uniformly across the substrate surface. SOGs are initially in a viscous liquid state, which allows the SOG to fill any open spaces, crevices, or voids which might be on the substrate surface. The coated substrate is then baked to remove water and solvents and to densify the SOG into a solid, glass-like film.
Conventional SOGs can be classified into two general categories, silicate SOGs and siloxane SOGs. Silicate SOGs are polymer networks containing primarily Si-O bonds. Silicate SOGs have an advantage of forming highly pure SiO.sub.2 films, without organic contaminants. A disadvantage associated with silicate SOGs is that upon baking, the films shrink considerably, creating high stresses in the films and in an underlying substrate. Another disadvantage is that silicate SOGs have poor step coverage, or in other words do not planarize uniformly or sufficiently. For example, a silicate SOG may planarize a small space between two structures, but as the size of the space increases the planarization capability is reduced. Siloxane SOGs, on the other hand, are polymer networks containing Si-O bonds and either Si-C bonds or Si-O-C bonds. Siloxane SOGs are an improvement in that siloxane SOGs shrink less upon baking and provide better step coverage than silicate SOGs. However, siloxane SOGs have a disadvantage of having a high organic content due to the presence of Si-C and Si-O-C bonds. The presence of organics in a SOG is unfavorable because organics can contaminate the film and affect performance and reliability of the film. While some of the organics can be removed by baking the film at higher temperatures or for longer periods of time, these remedies are also unfavorable because stresses in the film are consequently increased.
In an effort to improve planarization capabilities of SOGs, semiconductor manufacturers have used several techniques. One technique used is to apply a very thick SOG film. However, very thick films have a greater tendency to crack at points of high stress, particularly upon baking. Another technique is to apply multiple, thin SOG films to improve planarity without experiencing cracking problems. While several thin films can improve planarity with greater resistance to cracking, using multiple films significantly increases manufacturing time. In addition, multiple layer films tend to peel due to poor adhesion between the various layers. Another way to improve crack resistance of a SOG is to add dopants, such as boron or phosphorus, to the film. The bonding structure of a doped SOG seems to improve film strength, thereby reducing the susceptibility to cracking. However, there are applications in semiconductor devices in which doping a SOG is not appropriate.
Therefore a need exists for an improved spin-on-glass (SOG), and more specifically for an improved SOG for use in semiconductor fabrication which contains essentially no organics, provides good step coverage with minimal shrinkage after baking, is resistant to cracking, and can planarize sufficiently with one or more application.